The present invention relates to a capacitive load drive circuit used as a drive circuit for sustain electrodes and scan electrodes of a plasma display apparatus and to a plasma display apparatus that comprises a capacitive load drive circuit used as a drive circuit of sustain electrodes and scan electrodes.
The plasma display apparatus has been put to practical use as a flat display and is a thin display with high luminance. FIG. 1 is a diagram that shows the general structure of a conventional three-electrode AC-driven plasma display apparatus. As shown schematically, the plasma display apparatus comprises a plasma display panel (PDP) 1 composed of two substrates, between which a discharge gas is sealed, each substrate having plural X electrodes (X1, X2, X3, . . . , Xn) and Y electrodes (Y1, Y2, Y3, . . . , Yn) arranged adjacently by turns, plural address electrodes (A1, A2, A3, . . . , Am) arranged in the direction perpendicular thereto, and phosphors arranged at crossings, an address driver 2 that applies an address pulse to the address electrode, an X common driver 3 that applies a sustain discharge pulse to the X electrode, a scan driver 4 that applies a scan pulse sequentially to the Y electrode, a Y common driver 5 that supplies a sustain discharge pulse to be applied to the Y electrode to the scan driver 4, and a control circuit 6 that controls each section, and the control circuit 6 further comprises a display data control section 7 that includes a frame memory and a drive control circuit 8 composed of a scan driver control section 9 and a common driver control section 10. The X electrode is also referred to as the sustain electrode and the Y electrode is also referred to as the scan electrode. As the plasma display apparatus is widely known, a more detailed description of the entire apparatus is not given here and only the X common driver 3 and the Y common driver 5 that relate to the present invention are further described. The X common driver, the scan driver and the Y common driver of the plasma display apparatus have been disclosed, for example, in Japanese Patent No. 3201603, Japanese Unexamined Patent Publication (Kokai) No. 9-68946 and Japanese Unexamined Patent Publication (Kokai) No. 2000-194316.
FIG. 2 is a diagram that shows an example of the structure of the X common driver, the scan driver and the Y common driver, which have been disclosed as described above. The plural X electrodes are connected commonly and driven by the X common driver 3. The X common driver 3 comprises output devices (transistors) Q8, Q9, Q10 and Q11, which are provided between the common X electrode terminal and a voltage source +Vs1, between that and −Vs2, between that and +Vx, and between that and the ground (GND), respectively. By turning on any one of the transistors, the corresponding voltage is supplied to the common X electrode terminal.
The scan driver 4 is composed of individual drivers provided for each Y electrode and each individual driver comprises transistors Q1 and Q2, and diodes D1 and D2 provided in parallel thereto, respectively. Each of one end of transistors Q1 and Q2, and diodes D1 and D2 of each individual driver is connected to each Y electrode and each of the other end is connected commonly to the Y common driver 5. The Y common driver 5 comprises transistors Q3, Q4, Q5, Q6 and Q7, which are provided between the lines from the scan driver 4 and the voltage sources +Vs1, −Vs2, +Vwy, +Vy, and the ground (GND), respectively, and the transistors Q3, Q5, and Q7 are connected to the transistor Q1 and the diode D1, and the transistors Q4 and Q6, to the transistor Q2 and the diode D2.
In a reset period, Q5 and Q11 are turned on while the other transistors are being kept off, and +Vwy is applied to the Y electrode and 0V is applied to the X electrode to generate an entire write/erase pulse that brings the display cells in the panel 1 into a uniform state. At this time, the voltage +Vwy is applied to the Y electrode via Q5 and D1. In an address period, Q6, Q7, and Q10 are turned on while the other transistors are being kept off, and +Vx is applied to the X electrode, the voltage GND, to the terminal of Q2, and −Vy is applied to the terminal of Q1. In this state, a scan pulse that turns Q1 on and turns Q2 off is applied sequentially to the individual drivers. At this time, in individual drives to which a scan pulse is not applied, Q1 is turned off and Q2 is turned on, therefore, −Vy is applied to the Y electrode, to which the scan pulse is applied, via Q1, GND is applied to the other Y electrodes via Q2, and an address discharge is caused to occur between the address electrode to which a positive data voltage is applied and the Y electrode to which the scan pulse is applied. In this way, each cell in the panel is put into a state according to the display data.
In a sustain discharge period, while Q1, Q2, Q5 to Q7, Q10 and Q11 are being kept off, Q3 and Q9, and Q4 and Q8 are alternately turned on. These transistors are called the sustain transistors, wherein Q3 and Q8 that are connected to a high potential side power source are called the high-side switches, and Q4 and Q9 that are connected to a low potential side power source are called the low-side switches here. In this way, +Vs1 and −Vs2 are alternately applied to the Y electrode and the X electrode and a sustain discharge is caused to occur in the cell in which an address discharge has been caused to occur in the address period and the display is performed. At this time, if Q3 is turned on, +Vs1 is applied to the Y electrode via D1, and if Q4 is turned on, −Vs2 is applied to the Y electrode via D2. In other words, the voltage Vs1+Vs2 is alternately applied to the X electrode and the Y electrode, with a reversed polarity, in the sustain discharge period. This voltage is called the sustain voltage here.
The example described above is only one of various examples, and there are various modifications as to which kind of voltage is applied in the reset period, the address period, and the sustain discharge period, and there are also various modifications of the scan driver 4, the Y common driver 5 and the X common driver 6. Particularly in the drive circuit described above, +Vs1 and −Vs2 are applied alternately to the Y electrode and the X electrode to apply the sustain voltage of Vs1+Vs2=Vs, but there is another method in which Vs and GND are applied alternately and it is widely used.
In the general plasma display apparatus, the voltage Vs is set to a value between 150V and 200V, and the drive circuit is made up of transistors of large voltage rating (breakdown voltage). Contrary to this, in the driving method disclosed in such as Japanese Patent 3201603, Japanese Unexamined Patent Publication (Kokai) No. 9-68946 and Japanese Unexamined Patent Publication (Kokai) No. 2000-194316, the positive and negative sustain voltages (+Vs/2 and −Vs/2) are applied alternately to the X electrode and the Y electrode. This has an advantage in that it will be possible to reduce the breakdown voltage of the smoothing capacitor of the power source that supplies the sustain voltage.
The scan pulse must be applied sequentially to each Y electrode, therefore, Q1 and Q2, that relate to the application of the scan pulse, are required to be capable of high-speed operations. Moreover, as the number of times a sustain discharge is caused to occur affects the display luminance and as many sustain discharges as possible must be caused to occur in a fixed period, the sustain transistors Q3, Q4, Q8, and Q9, which relate to the application of the sustain discharge pulse, are also required to be capable of high-speed operations. On the other hand, in the plasma display apparatus, it is necessary to apply a high voltage to each electrode in order to cause a discharge to occur, therefore, the transistors are required to have a high breakdown voltage. A transistor which has a high breakdown voltage but has a relatively low operating speed, or a transistor which has a high operating speed but has a relatively low breakdown voltage, can be manufactured at a low cost, but a transistor which has not only a high breakdown voltage but also a high operating speed is costly.
Among the transistors in FIG. 2, the operating speed of Q6, Q7, Q10 and Q11 can be relatively low because they do not directly relate to the application of the scan pulse and the sustain discharge pulse, which requires a high-speed operation. Although a high-speed operation is required for Q1 and Q2, their breakdown voltages can be relatively small, because D1 and D2 are provided in parallel thereto, the voltages to be applied are −Vy and GND, and the difference in voltage therebetween is relatively small.
Contrary to this, the sustain transistors Q3, Q4, Q8, and Q9 need to be capable of high-speed operations and a high voltage is applied thereto as well. Among the applied voltages in the circuit in FIG. 2, the largest is the reset voltage +Vwy and the smallest one is −Vs2. When Q5 is turned on and the reset voltage +Vwy is applied, therefore, the voltage Vwy+Vs2 is applied to the sustain transistor Q4, as a result. Normally, −Vy is greater than −Vs2 (the absolute value is less) and +Vx is less than +Vs1. The maximum voltage to be applied to other sustain transistors Q3, Q8 and Q9 is Vs1+Vs2, which is less than the voltage Vwy+Vs2 to be applied to Q4.
As described above, there are various modification examples of the voltage to be supplied from the drive circuit of the plasma display apparatus, therefore, the maximum voltage to be applied to each sustain transistor differs from another accordingly. In general, when a voltage greater than the sustain voltage on the high potential side is applied, the maximum voltage to be applied to the sustain transistors that make up the low-side switch is greater than the sustain voltage, and when a voltage less than the sustain voltage on the low potential side is applied, the maximum voltage to be applied to the sustain transistors that make up the high-side switch is greater than the sustain voltage.
In the conventional apparatus, sustain transistors of the same breakdown voltage (voltage rating) are selected despite the difference in the maximum voltage to be applied, as described above. In other words, the devices are selected so that their breakdown voltages correspond to that of the sustain transistor that receives the maximum voltage, and other sustain transistors are selected from those of the same breakdown voltage. This means that different kinds or sizes of transistors are selected when devices of different breakdown voltages are selected and, as a result, the switching performance of each transistor is different. Moreover, a device of a high breakdown voltage has a high saturation voltage and a circuit structure is required in which plural devices are driven in parallel in order to lower the saturation voltage. If, therefore, sustain transistors of different breakdown voltages are used, the switching performance of each sustain transistor differs from another and a problem is caused in that they cannot be turned on/off stably. In the sustain (sustain discharge) action, charges are moved from one electrode to the other and the timing of the application of the sustain voltage is important and, therefore, a problem that the sustain action is terminated is caused if the timing is not correct.
For the above-mentioned reasons, a capacitive additional drive circuit, such as that of the sustain electrode and that of the scan electrode of the plasma display apparatus, is not configured by combining drive transistors (output devices) of different breakdown voltages.
On the other hand, in the conventional plasma display apparatus, the sustain voltage is supplied by applying GND to one of the electrodes but a structure in which the breakdown voltage of the smoothing capacitor of the power source, that supplies the sustain voltage, can be lowered by applying the positive and the negative voltages alternately to the X electrode and the Y electrode as described above, has been disclosed in Japanese Patent No. 3201603, Japanese Unexamined Patent Publication (Kokai) No. 9-68946 and Japanese Unexamined Patent Publication (Kokai) No. 2000-194316. In order to apply the sustain voltage in the way described above, a compact power source circuit that can stably supply positive and negative voltages with high precision is required.